Why is my cat's stool bloody after deworming, no parasites?

I'm sorry Apple isn't doing well at the moment! There are a number of other causes of blood in the stool such as a bacterial infection, viral infection, a foreign body, eating something rotten, giardia, coccidia, or damage to the GI tract among other causes. I'm not sure if the vet did a fecal float to check to check for parasites, but giardia cysts are often not detected with this test because of their small size, and will often require a special zinc sulfate flotation solution for detection. You can ask your vet if they have done this test to check for giardia as a possible cause of her bloody stool. Further tests may be needed as well such as blood work, a urinalysis, x-ray of her abdomen, or an ultrasound of her abdomen. I would speak to your vet to see if one of these tests could be done to help diagnose the cause of her symptoms so she can be treated. I hope this helps, best of luck!